Jet Drive problem

Discussion in 'Jet Drives' started by Orie, Dec 20, 2017.

  1. Orie
    Joined: Dec 2017
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    Location: Trinidad & Tobago

    Orie New Member

    Possible causes of damages to Jet drive guide vane chamber
     

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  2. Barry
    Joined: Mar 2002
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    Barry Senior Member

    As all the vanes appear to be impacted by the same type of erosion, it looks like the pumps have been running in extremely sandy water. If only one or two vanes had lost material, then
    perhaps a rock had gone through them
    These are called stators and are at the trailing end of the impeller but fixed to the housing. The water coming off the impeller is not 100% axial, it is not moving parallel to the shaft. This water has momentum and when the water hits the stator, the stator tries to direct the water back to an axial path. As Force = mass times acceleration and acceleration is a change in velocity.
    Velocity is a direction and speed. So when you change the direction, though the speed of the water is the same, there is an acceleration of the water, (change in velocity) and hence you
    get some "free" thrust.
    What kind of pumps are these?
     
  3. Orie
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    Orie New Member

    These are Rolls Royce Kamewa FF375 water jet drives ,Could this be as a result of Cavitation
     
  4. Barry
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    Barry Senior Member

    On closer inspection, it most likely is. I did not look closely at 122756 picture and it appears that the inlet side of the vanes do not appear to be eroded which would have supported
    the erosion factor. As it appears that the trailing surface is exhibiting most of the material loss and the sides of some of the stator hubs do not appear to be impacted, some of them,
    it is probably cavitation.
    The question I should have asked initially was " are the jets used in rivers or on the ocean/fresh water"

    I have seen many stators eroded as the jet boats that we built most always spent 90% of their time in river water and many on the larger rivers that carry a large sediment load.
    sorry if I began to lead you away from an obvious reason

    The extremely rough surface is indicative of cavitation corrosion whereas if the material was eroded by sand, the surfaces should have been smooth.
     
  5. Orie
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    Orie New Member

    Barry your comments are welcome
    The vessel is operated in the Gulf of Paria I a minimum of 30 feet salt water
     
  6. anthony goodson
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    anthony goodson Senior Member

    Looks like straightforward electrolysis to me ,I can't see any sign of cavitation on the impellers
     
  7. Barry
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    Barry Senior Member

    Because the softer aluminum has suffered cavitation corrosion, does not mean that the bronze/brass impeller has to exhibit cavitation corrosion. The impellor is much harder and the flow
    regime over the blades may be such that conditions for water to vaporize (cavitation) and implode taking material with it (cavitation corrosion) may not be present.
    In fact the impeller may be creating the bubbles that form but at the trailing edge of the blades, the bubbles shed off the blade, contacting the softer aluminum stators, then imploding
    upon the stator when the pressure begins to increase.
     
  8. Barry
    Joined: Mar 2002
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    Barry Senior Member

    [​IMG]
    Vapor bubble forming on the tips of a prop
    [​IMG]
    Cavitation the creation of bubbles in a flow
    Cavitation Corrosion, the erosion of material caused when the bubbles implode against a surface, down stream
     
  9. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    Assuming the damage started with corrosion, the rough leading edge will cause cavitation further increasing the damage.
     
  10. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    Nope, this damage has started with cavitation, there are all the typical marks of cavitation to be seen both on the stator blades and where blades join the outer surface. On the Picture with the Collection of stators you can see that the cavitation implosion zone has a direction that indicates a too high angle of attack relative to the leading edge direction. The stator blading is overloaded and cannot turn the residual vortex into a longitudinal flow; the result is damage and low efficiency. There is something basically wrong with the matching of jet, hull and Engine.

    Once the Surface protection is gone (takes less than an hour) there is serious corrosion coming into play; the aluoxide formed on the alu is quickly hammered away by the cavitation, so there is an open area for galvanic currents flowing in the cirquit SS impeller>alu housing>bearing housing>bearings>shaft>back to impeller.

    I'd like to know more about power, rpms and operating speeds in particular, but also a few facts about the boat. I've seen similar damage on FF jets before, and there may be a "medicine" to apply. What is the RR response to this issue?
     
  11. gonzo
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    gonzo Senior Member

    I doubt you can determine which was first from a photo alone. Even analyzing the material it would be hard.
     
  12. baeckmo
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    baeckmo Hydrodynamics

    ...and how, exactly would you explain the corrosion inception to take place if nothing else had broken the surface protection first?
     
  13. gonzo
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    Location: Milwaukee, WI

    gonzo Senior Member

    Cavitation can damage the impeller and that will lead to the start of corrosion. The corroded leading edge will increase cavitation.
     
  14. Orie
    Joined: Dec 2017
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    Location: Trinidad & Tobago

    Orie New Member

    RR response firstly was overloading but the vessel only carry two to three personnel to vessels then they suggest cavitation . Vessel operated in minimum 30 feet of salt water in the GULF OF PARIA ,is it because of low draft of vessel ?attached is vessel of same type and specification ,also attached is a pic of initial stage of GVC cavitation .
     

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  15. baeckmo
    Joined: Jun 2009
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    Location: Sweden

    baeckmo Hydrodynamics

    This confirms what I said before; cavitation is the cause; there is no erosion and the corrosion is a secondary damage. Now five questions:
    1) Could you provide one more picture of the above stator, but rotated 180 degrees, and please indicate top of stator as installed.
    2) Could you confirm the weight of the vessel? My guesstimate is about 18 tonne, am I far off?
    3) Could you find the diameter of the outlet nozzle at the stator exit (not the steering nozzle!!)?
    4) Dou you have any pictures showing the jet inlet and the attachement to the hull?
    5) What is the transmission gearing ratio (need to know the rotating speed of the jet impellers)?

    And Anthony: glad to see you are still around, hope you are ok; these impellers are very good quality austenitic steel, with excellent cavitation resistance. If the leading edge aoa is low, and if the impeller loading is high, the cavitation damage will occur in the stator, just as seen here. If I can get the answers on the above questions, I may be able to suggest a medicine.
     
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